1. Field of the Invention
This invention relates to an original illuminating device which illuminates an original with light-emitting devices, and which is used in a facsimile, a copier or the like. The invention also relates to an original reading device which reads light from an illuminating device with a reading member.
2. Description of the Related Art
Heretofore, light sources emitting a large amount of light, such as fluorescent tubes, xenon tubes and the like, have been used as light sources for image reading apparatuses, such as facsimile machines and the like. Recently, however, since photosensors, such as CCDs (charge-coupled devices) and the like, tend to have higher sensitivity, light sources comprising light-emitting devices emitting a small amount of light, specifically, LEDs (light-emitting diodes) arranged in the shape of an array have appeared in the market.
This is because the LED has, for example, the following features. The LED can be provided in a small size and with low cost since it does not need an inverter for lighting it as do fluorescent tubes and xenon tubes. The LED does not generate high-frequency noise since it does not need high-frequency lighting. Furthermore, the LED has more stable rising characteristics and temperature characteristics than fluorescent tubes and xenon tubes.
An explanation will now be provided of a device using LEDs.
FIG. 7 shows the schematic configuration of an original reading device 100, to describe a background technique of the present invention.
In FIG. 7, there are shown an upper original mount 101, a white reference background 102 provided at the lower surface of the upper original mount 101, lower original mounts 103 and 104, and an original-mount glass 105. Below the original-mount glass 108 are disposed a substrate 106, a reflecting plate 112, and a photosensitive member, such as a photoelectric conversion device or the like (not shown). The substrate 106 faces reading position A of an original in a state inclined about 45.degree. relative to the original-mount glass 105.
FIG. 8 is a perspective view of the substrate 106.
On an identical surface of the substrate 106 there are linearly arranged LED chips 107, serving as a plurality of light-emitting devices, disposed in the direction of the longitudinal direction, and there are also arranged, in parallel, electrodes (made of a metal, such as gold, silver, A1 or the like) 108 for passing current corresponding to the respective LED chips 107. The electrodes 108 are situated at positions so as to pass between the LED chips 107 and enclose the LED chips from both sides in the direction of the width of the substrate 106 orthogonal to the longitudinal direction thereof. Each of the electrodes 108 is connected to the corresponding LED chip 107 using a bonding wire 108a. There are also shown resistors 109, and patterns 110 and 111 for passing current to the substrate 106.
The LED chips are electrically connected so that a plurality of (eight in the case of FIG. 8) LED chips are connected in series using the electrodes and the bonding wires to form a group, and respective groups are connected in parallel using a circuit pattern. Since LED chips within one group are connected in series, respective electrodes are arranged while being separated with a space l.
In the original reading device 100 configured as described above, current is supplied to the substrate 106 from the outside to cause the LED chips 107 to illuminate the reading position A of the original D through optical path E shown by one-dot chain lines, as shown in FIG. 7. The light reflected by the original D is guided to the photosensitive member through optical path B, and an image on the original D is read.
The above-described device, however, has the following disadvantages, since the LED chips 107 and the electrodes 108 are provided on the same surface of the substrate 106.
It can be considered that each LED chip 107 is a point light source around the reading position A. Illuminating light from each LED chip 107 is randomly reflected by surrounding components in a complex manner to illuminate the entire area. Such randomly-reflected light reaches the electrodes 108 and illuminates the reading position A as a secondary light source, thereby adversely influencing a reading operation by the photosensitive members.
That is, the reflected light which has reached the electrodes 108 situated at the side of the optical path B relative to the LED chips 107 reaches the reading position A via optical paths E, M and N, as shown in FIG. 7. In this case, the light beams along the optical paths N and M illuminate the reading position A from a direction closer to the perpendicular direction relative to the original-mount glass 105 than the light beam along the optical path E. Hence, according to a certain setting of the optical paths, such light, serving as a secondary light source, is specularly reflected by the lower and upper surfaces of the original-mount glass 105, and is incident upon the CCD via a mirror optical system and a lens, acting in some cases as white noise which is different from the light reflected from the surface of the original D. Such a white-noise component is in some cases as large as about 10% -20%. As a result, the randomly-reflected light causes a failure in a reading operation of an image on the original; for example, the original D is determined to be white while it is actually black.
As described above, the electrodes 108 on the substrate 106 are not continuous along the longitudinal direction, but are arranged in a mosaic pattern having a gap l. Hence, the above-described white noise caused by the secondary light source has variations in its magnitude in the longitudinal direction of the substrate. As a result, an undulation as indicated by a broken line in FIG. 5 appears in the output from the photosensitive member even if the original D is totally white.
Next, an explanation will be provided of a case wherein a condenser lens is provided incident on the LED chips in order to increase the amount of light on the reading position A. FIG. 16 illustrates the schematic configuration of an original reading device, to describe a background technique of the present invention.
In FIG. 16, there are shown an upper original mount 201, a white reference background 202 provided at the lower surface of the upper original mount 201, lower original mounts 203 and 204, and original-mount glass 205. Below the original-mount glass 205 are disposed a substrate 206, and a photosensitive member, such as a photoelectric conversion device or the like (not shown). The substrate 206 faces illuminating position A of an original in a state inclined about 20.degree.-40.degree. relative to the original-mount glass 205.
On the upper surface of the substrate 206 are linearly arranged LED chips 208, serving as a plurality of light-emitting devices, disposed in the longitudinal direction thereof (the direction of the width of the original). A condenser lens 209 having condensing surfaces 209a and 209b is disposed above the LED chips 208. The condenser lens 209 may be provided in various manners. For example, each condenser lens 209 may be disposed for each LED chip 208. Alternatively, a rod-like condenser lens 209 may cover the entire row of the LED chips 208. Optical path 301 may be inclined in a direction separate from the LED chips 208 relative to the original-mount glass 205 in order to obtain a necessary amount of light by disposing the substrate 206 closer to the reading position A. The substrate 206 is secured to a holding member 211 using a lock screw 210.
In an original reading device 200 configured as described above, the LED chips 208 emit light to illuminate the reading position A of the original through optical path 300 shown by the one-dot chain line. The light reflected by the original,is guided to the photosensitive member through the optical path 301, and an image on the original is read.
The above-described device, however, has the following disadvantages, since the condenser lens 209 is disposed above the LED chips 108.
In the above-described original reading device 200, each LED chip 208 can be considered as a point light source. The light emitted from the LED chips 208 illuminates the original reading position A via the condensing surfaces 209a and 209b of the condenser lens 209. Part of the light, however, is randomly reflected by condensing portions and sides of the condenser lens 209 not directly related to the condensing function, and mounting portions of the condenser lens 209 within the condenser lens 209. Particularly when such randomly-reflected light leaks toward the original-mount glass 205 from the side closer to the optical path 301 than the LED chips 208, the light illuminates the illuminating position A from a direction closer to the vertical direction relative to the surface of the original-mount glass 205 than the direct light from the LED chips 208. That is, the light emitted in direction T.sub.1 enters the condenser lens 209 from a surface other than the condensing surface 209a, and is directed in direction T.sub.2. The light is then directed in direction P after repeated reflection along a path indicated by two-dot chain lines, and is specularly reflected by the lower surface of the original-mount glass 205 to enter the optical path 301, functioning as noise for an image reading signal and causing a failure in an image reading operation. Another light beam in direction Q is also present, and the light is specularly reflected by the upper surface of the original-mount glass 205, causing the same failure. Such a noise component is in some cases as large as 10-20% of the light reflected by the original.
Furthermore, since variations are present in the magnitude of the noise in accordance with the arrangement of the row of the LED chips 208, an undulation in accordance with the row of the LED chips, the same as the undulation shown in FIG. 5, appears in the output from a CCD sensor, serving as a photosensitive member. As a result, vertical stripes having the same pitch as that of the row of the LED chips 208 appear in a read image.
It is possible to eliminate the influence of such undulations by sufficiently separating the LED array from the reading position. In this case, however, the amount of light from the LED chips cannot be sufficiently utilized. Hence, it is necessary to increase the number of the LED chips, causing an increase in the production cost. Another problem is that it is necessary to provide a space for separating the substrate having the LED array thereon from the reading position and hence the size of the device is increased.
The above-described disadvantages are further increased for the light randomly reflected by the electrodes explained with reference to FIGS. 7 and 8, if the condenser lens shown in FIG. 16 is provided. That is, light having small incident angles from the LED chips is reflected by the incident surface of the condenser lens, is then reflected by the electrodes on the substrate, and specularly illuminates the original-mount glass after passing through an end portion of the condenser lens. Since this light is superposed with the directly-illuminating light from the LED chips, it becomes noise, further increasing the degree of failure in an image reading operation.